Your power bank is lying to you about its capacity - sort of

Many consumers misunderstand power bank capacity, believing a 10,000 mAh battery can fully recharge a smartphone with a 4,000 mAh battery twice. In reality, discrepancies occur due to voltage differences and energy loss during the charging process.

Power banks primarily output at 5V, while their internal batteries function at approximately 3.7V. This difference affects the actual usable capacity and leads to a discrepancy between labeled and real-world performance.

Read at ZDNET